As a result of dwindling energy supplies much effort is being put into perfecting apparatus for collecting solar energy and putting it to use, for example, to heat water for domestic or industrial use or as a heat storing medium. Since solar heat collectors are often exposed to ambient temperatures well below the freezing temperature of water, it is attractive to provide a dual-liquid solar heating system wherein the water flows in one circuit which is protected inside a building or buried, and wherein the system has another circuit in which a non-freezing liquid circulates through a solar collector which is exposed to out-of-doors temperatures. Oil can be used in this exposed circuit, or ethylene glycol, or some other such liquid, although there is more to the selection of the liquid than its antifreeze qualities. For example one must consider its thermodynamic properties such as thermal conductivity, its viscosity when cold, anticorrosion properties, cost, etc. One of the more attractive liquids is silicone oil. However, silicone oil is considerably more viscose than water and therefore tends to form a relatively motionless boundary later, which tends to be broken up by the fins when the oil flows over finned tubes in a heat exchanger. Moreover, silicone oil might contaminate a domestic water supply if a leak were to occur within the heat exchanger. The present invention addresses itself to providing solutions to these, and other problems, by providing an improved heat exchanger structure.
Prior patents known at the present time show heat exchangers of the type having a shell through which one fluid flows and having within the shell a circuitous fluid path extending around a core member and passing a second liquid, examples thereof being shown in Canadian Pat. No. 468,323 to Hill; U.S. Pat. No. 3,802,499 to Garcea, and U.S. Pat. No. 3,092,980 to Rosenschold. The latter two patents show the circuitous path as comprising a helical coil of finned tubing with a core member closing the central opening through the helix.
In one major application for which the present heat exchanger is specifically provided, namely solar energy collection, a double-walled tube must be used as a government-required safety measure to contain the domestic water and separate it from the liquid medium, such as silicone oil which collects heat from the solar panels. A number of patents are known which show straight-tube double-walled structures, for instance, as shown in U.S. Pat. No. 2,187,555 to Flindt, U.S. Pat. No. 2,365,515 to Baudry, and British Pat No. 748,264 to Foster-Wheeler. These double-walled tubes make it possible to guard against contamination of one liquid by the other in case one tube should leak, and they also provide drippage at the end of the outer tube to indicate any such leakage.
In solar heat exchange service, however, the use of double-walled tubing in a heat exchanger is very damaging to its heat transfer rate, particularly so because the temperature of the liquid from the solar panels is not very much higher than the temperature to which the water inside the double-walled tubing is to be heated. In a typical solar installation the medium bringing heat from the solar panels will have a difference in temperature with respect to the water in the volute tubes of only about 40.degree. F. The problem of poor heat transfer occurs because the space between the dual tubes, if sufficient to channel leakage outside the heat exchanger acts as an insulating barrier, and this insulating tendency is intolerable because of the low differential in temperatures between the silicone oil in the outer fluid circuit and the water on the other side of the dual tubing wall. The present invention seeks to minimize the deleterious effect of this insulating space by improving the metal-to-metal physical contact between the tubes as a direct result of winding them into a helical volute of small diameter whereby the tubes are distorted into mutual contact.
The prior patents mentioned above are all of record, with copies supplied, in the parent case of which this disclosure is a C.I.P.